As one of the key components in RF front-end devices, a voltage controlled oscillator (VCO) with low phase noise is highly demanded. Injection locking is a technique that is used to reduce the phase noise [1]-[4] of a VCO. It is a type of forced oscillation where a signal is injected into the oscillator and “locks” the oscillator to the frequency and phase of the injected signal under proper conditions.
Self-injection locking is a special type of the injection locking technique, where the injected signal comes from the oscillator itself instead of from an external source. An early self-injection-locking arrangement is shown in FIG. 1 [1], [2], where a circulator is used to direct the signal propagation in a desired way [2]. T. P. Wang et al proposed another self-injected push-push oscillator [3]. In their implementation, the second harmonic signal of the oscillator arrangement is used for self-injection locking. Recently, U. L. Rohde and A. K. Poddar proposed a self-injection locking setup based on two coupled VCOs, where the output signal from one VCO is injected into the other.
European Patent Application EP 1713182 discloses an oscillator array with a plurality of series-connected oscillators, and at least one band elimination filter, which is disposed between respective adjacent two of the oscillators. In addition, a synchronization method for oscillator array is disclosed, in which a signal generated from each of the oscillators is in phase with a signal reflected by corresponding band elimination filter at elimination frequencies of the band filter, and in opposite phase with signals leaked from corresponding band elimination filters.
In the above-mentioned known solutions, the output signal is split into two and one of them is delayed via signal propagation through the delay line and then is injected into the oscillator. The delay must be sufficiently long, so that the delayed signal is uncorrelated with the un-delayed signal [1, 5]. Typically, several nano seconds delay is required for GHz oscillators. This makes it difficult to implement the delay line on a substrate, such as Si or glass substrate, since the resulting component would be too large and too costly.